1. Field of the Invention
This invention relates to flueric partial pressure sensors and is more particularly concerned with a flueric partial pressure sensor for use in controlling a molecular sieve type gas separation system in an on-board oxygen enrichment of air system for an aircraft.
2. Description of the Prior Art
U.S. Pat. No. 4,407,153 which corresponds to EP-A-0 036 285 (Normalair-Garrett) discloses a flueric partial pressure sensor which may be incorporated in a molecular sieve type gas separation system (MSOGS) for delivering oxygen-enriched air to aircrew members, an example of such a system being disclosed in and EP-A-0 129 304 (Normalair-Garrett) which corresponds to pending U.S. patent application Ser. No. 595,303 filed 30 Mar., 1984.
In aircraft capable of long range flight at altitudes up to, say, 60,000 feet (18,290 meters), it is desirable and practicable, for the comfort and efficiency of the crew, to so pressurise the aircraft cabin that the cabin pressure does not fall below an altitude equivalent of, say, 8,000 feet (2,440 meters) up to a chosen aircraft altitude - say 45,000 feet (13,720 meters) - and thereafter to maintain the same cabin to atmosphere pressure difference with increasing aircraft altitude.
Whilst failure of cabin pressurisation equipment is a remote possibility provision must be made for possible depressurisation which, at worst, could be instant. In anticipation of instant depressurisation it is expedient, medically, to increase the oxygen concentration in the air breathed by the crew when the aircraft is above a certain altitude. Thus, whilst the cabin pressure may be held at an 8,000 feet (2,440 meters) equivalent up to the chosen altitude of, say, 45,000 feet (13,720 meters), the oxygen concentration in the air delivered from the breathing system should be made to increase beyond the 8,000 feet (2,440 meters) equivalent concentration value - about 29% - as the aircraft climbs above an aircraft altitude of, approximately, 30,000 feet (9,145 meters).
Furthermore, a flueric partial pressure sensor as disclosed in U.S. Pat. No. 4,407,153 will operate only at absolute pressures up to an altitude equivalent of about 25,000 feet (7,620 meters), pressure above this altitude being insufficient to drive the sensor.